CVE-2026-89682
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
nfsd: fix fcache_disposal UAF by inlining dispose state into nfsd_net
nfsd_file_dispose_list_delayed() defers fput() to nfsd service threads via a per-net freeme queue, preventing the shrinker and GC worker from bearing the cost of closing files (see ffb402596147). However, the queue lives in a separately-allocated struct nfsd_fcache_disposal that is freed by nfsd_free_fcache_disposal_net() during per-net teardown. The global shrinker, laundrette, and fsnotify callbacks can still be inside nfsd_file_dispose_list_delayed() dereferencing that pointer, causing a use-after-free.
Inline the spinlock and freeme list directly into struct nfsd_net (as fcache_dispose_lock and fcache_dispose_list), eliminating the separately allocated struct nfsd_fcache_disposal entirely. These fields now have the same lifetime as the net namespace itself, so there is no dangling pointer to chase.
nfsd_file_cache_start_net() now just initializes the inline fields and cannot fail due to allocation. nfsd_file_cache_shutdown_net() drains the inline list directly instead of freeing a separate struct. The alloc/free helpers are removed.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability identified in the Linux kernel's NFS server implementation involves a critical use-after-free condition within the file cache disposal mechanism, specifically affecting the nfsd service threads and associated background workers. This flaw originates from an architectural design choice where the deferred file put operations were managed through a separately allocated structure known as struct nfsd_fcache_disposal. The primary intent behind this separation was performance optimization; by deferring fput calls to dedicated nfsd service threads via a per-network namespace freeme queue, the system prevents the global shrinker and garbage collection worker from incurring the heavy cost of closing files directly. This design aims to maintain responsiveness during high-load scenarios where file descriptor cleanup might otherwise block critical kernel operations.
However, this separation introduced a race condition related to memory lifecycle management. The separately allocated struct nfsd_fcache_disposal is freed by the function nfsd_free_fcache_disposal_net() during the teardown of per-network namespaces. Despite this deallocation, other components such as the global shrinker, the laundrette worker, and fsnotify callbacks can still be executing within the context of nfsd_file_dispose_list_delayed(). These functions continue to dereference pointers pointing to the now-freed disposal structure, resulting in a classic use-after-free vulnerability. This scenario allows for potential memory corruption or arbitrary code execution if an attacker can trigger network namespace teardown while these background processes are active and accessing the stale pointer.
The remediation strategy involves refactoring the data structures to eliminate the separate allocation entirely by inlining the spinlock and freeme list directly into struct nfsd_net as fcache_dispose_lock and fcache_dispose_list. By embedding these fields within the network namespace structure, their lifetime becomes synchronized with that of the net namespace itself. This ensures that there is no possibility for a dangling pointer to exist because the memory referenced by background workers remains valid until the entire network namespace is destroyed. Consequently, the functions nfsd_file_cache_start_net and nfsd_file_cache_shutdown_net have been updated to initialize and drain these inline fields directly, removing the need for separate allocation and deallocation helpers that previously introduced the window of vulnerability.
From a classification perspective, this issue aligns with CWE-416, Use After Free, where memory is accessed after it has been freed, leading to undefined behavior. In terms of attack vectors, this could be leveraged in scenarios involving privilege escalation or denial of service by exploiting the race condition during network namespace manipulation. The ATT&CK framework would categorize potential exploitation techniques under T1059, Command and Scripting Interpreter, if used for lateral movement, or more broadly under system resource exhaustion tactics due to kernel instability caused by memory corruption.
To mitigate this vulnerability in environments where patching may not be immediately feasible, administrators should minimize the creation and destruction of network namespaces unless strictly necessary, as these operations trigger the teardown sequence that exposes the race condition. Additionally, monitoring for unusual spikes in nfsd service thread activity or errors related to file descriptor handling can provide early indicators of exploitation attempts. It is imperative to apply kernel updates that include this fix promptly, ensuring that the inline data structures are utilized correctly and that the separate disposal structure is no longer allocated or freed independently. Regular auditing of network namespace configurations and adherence to least-privilege principles for services interacting with NFS file handles will further reduce the attack surface associated with this class of memory management errors.